In PoDL, DC power from Power Sourcing Equipment (PSE) is transmitted over a single twisted wire pair. The same twisted wire pair also transmits/receives differential data signals. In this way, the need for providing any external power source for the Powered Devices (PDs) can be eliminated. The standards for PoDL are set out in IEEE 802.3 and are well-known.
A conventional PoDL system uses a coupling network to couple the DC power and AC data to the twisted wire pair at the output of the PSE and uses an identical network to decouple the DC power and AC data from the twisted wire pair at the PD.
FIG. 1 illustrates conventional coupling/decoupling networks between a PSE 10 and a PD 12 in an Ethernet PoDL system. The PSE 10 includes a DC voltage source 13 and may include a differential data transceiver. The differential data may also be generated by any other circuit. The differential data is applied to differential terminals of the physical layer (PHY) 14 for application to the twisted wire pair 16. The data portion of the PoDL system is not relevant to the present invention so is not described in detail.
The PD 12 includes a differential data portion that receives data from the PHY 18 terminals and processes the data accordingly. Such a data processing portion is not relevant to the invention. The PD load that receives the DC voltage and the data is represented by a resistor RPD. A capacitor CPD helps smooth the voltage into the PD load. A DC-DC converter may be used in the PD to convert the received PoDL voltage to a target voltage for the PD load.
In the example of FIG. 1, DC power is delivered from the PSE 10 to the PD 12 through the single twisted wire pair 16 via a coupling network that conducts DC (or low frequency current), for power, between the DC voltage source 13 and the wire pair 16, while simultaneously blocking the differential AC data (or high frequency current) from the DC voltage source 13. Similarly, the PD 12 uses a decoupling network that decouples the transmitted DC voltage for powering the PD load, while conducting only the PHYs' AC data to data terminals in the PD 12. The ability of the coupling/decoupling networks to block the PHYs' AC data over a very broad range of frequencies is a key requirement for PoDL Ethernet applications where the data rates may vary from 100 Mbps to 1 Gbps. In the example of FIG. 1, the capacitors C1-C4 are intended to block DC in the data path, while the inductors L1-L4 are intended to block AC in the power path.
In FIG. 1, inductors L1-L4 are used to couple/decouple the DC flowing between the PSE 10 voltage source 13 and the PD 12 load to/from the wires 16. The inductors L1-L4 are AC blocking devices whose impedance is proportional to frequency. The constant of proportionality is referred to as the inductance L. The ability of a single inductor to impede AC over a broad range of frequencies depends on the magnitude of inductance, the inductor's ability to conduct DC current without losing its inductance, and its parasitic capacitance.
It is desirable to make the inductors L1-L4 the minimum size necessary to pass the power signal but block the AC data signals. Similarly, it is desirable to make the capacitors C1-C4 the minimum size necessary to block the power signal but pass the AC data signals. However, dV/dt noise in the power signal must also be blocked, and such dV/dt noise is fairly unpredictable. The dV/dt noise may affect data integrity. Therefore, the inductors L1-L4 and capacitors C1-C4 are typically larger than required to adequately pass or block the DC voltage and pass or block the AC data signals. Noise in the power signal may arise while the PSE being turned on, or from other equipment on the power supply bus, or from other sources.
Similarly, a rapid change in the PD load current (dl/dt) affects the voltage delivered by the PSE, where a high positive dl/dt will cause a rapid temporary decrease in the voltage, and where a high negative dl/dt will cause a rapid temporary increase in the voltage. Such dV/dt changes in voltage may affect data integrity.
Thus, what is needed in the field of PoDL is an improved network that combines or separates the power signal and the wide bandwidth AC data while limiting noise in the power signal caused by dV/dt or dI/dt.